1. Field of the Invention
The present invention relates to a spin-torque oscillator, a magnetic head including the spin-torque oscillator, and a magnetic recording and reproducing apparatus.
2. Related Art
It is known that a steady-state microwave signal that responses to a direct current can be generated by taking advantage of the spin transfer effect caused in a nano-scale magnetic multi-layer film having a spacer layer interposed between magnetic layers (see “Microwave oscillations of a nanomagnet driven by a spin-polarized current”, S. I. Kiselev, et al., Nature 425, 380 (2003), for example). The origin of the microwave signal is an magnetization oscillation in a magnetization oscillating unit in the magnetic multi-layer film, and, on an experimental basis, a steady-state magnetization oscillation of high frequency (GHz) is detected when the current density exceeds the order of 107 A/cm2 in a CPP (Current Perpendicular to Plane)-GMR (Giant Magneto-Resistive effect) film or a magnetic tunnel junction (MTJ) film.
A microwave generator that uses the spin transfer effect in a magnetic multi-layer film is called a spin-torque oscillator. Thanks to the significant improvement in the microfabrication technology, it is possible to form a CPP-GMR film or a magnetic tunnel junction film of a submicron size such as 100 nm×100 nm. Spin-torque oscillators are expected to be applied to small-sized microwave sources and resonators, and are being actively studied in the field of spintronics. The frequency of a microwave signal generated from a spin-torque oscillator depends on the current and the magnetic field acting on the magnetization of the magnetization oscillating unit in the magnetic multi-layer film. Particularly, there has been a suggestion that spin-torque oscillators are used as HDD (Hard Disk Drive) magnetic sensors in place of GMR heads and TMR heads, taking advantage of the magnetic field dependency of the frequency of magnetization oscillations varying with a magnetic field (see JP-A 2006-286855(KOKAI), for example). According to the suggestion, a magnetic field on a HDD medium is detected by sensing changes in frequency caused by the magnetic field.
A conventional spin-torque oscillator is designed to extract a microwave signal generated from an oscillation of magnetization in a magnetoresistive element unit having a ferromagnetic multi-layer film. The magnetoresistive element unit has a three-layer structure as a fundamental structure formed with a magnetization free layer, a spacer layer, and a magnetization pinned layer. When a direct current I is supplied from a power supply to the ferromagnetic multi-layer film of the magnetoresistive element unit, the magnetization M in the magnetization free layer oscillates by virtue of the spin transfer effect caused between the magnetization free layer and the magnetization pinned layer, and the angle θ of the magnetization of the magnetization free layer with respect to the magnetization of the magnetization pinned layer varies with time. As the angle θ varies, the device resistance also varies with time due to a spin-valve magnetoresistive effect, and a high-frequency voltage appears. The high-frequency component is extracted by a bias tee, so as to obtain a microwave signal P as an output.
The direct current I supplied from the power supply cannot have any value, and needs to have a larger current value than the value of a threshold current Ic that depends on the structure of the magnetoresistive element unit having a ferromagnetic multi-layer film and on the magnetic field acting on the magnetoresistive element unit. A magnetization oscillation is caused in the magnetization free layer by virtue of the spin transfer effect, only if the direct current I is larger than the threshold current Ic. The value of the threshold current Ic is determined by the cross-sectional area of the magnetoresistive element unit and the value of the threshold current density. The value of the threshold current density is of the order of 107 A/cm2.
There is a so-called Q value (a quality factor) that represents the properties of an oscillator. To explain an example of a Q value, an oscillator circuit that uses a crystal oscillator as resonator is now described. A crystal oscillator is known to have a high Q value of the order of 106, and an oscillator circuit using a crystal oscillator as a resonator can have a Q value of the order of 103 to 104 to achieve stable oscillations. A Q value is a dimensionless quantity that is defined as: Q=energy stored in one cycle/power loss in one cycle (dissipation energy). As a Q value is larger, a more stable oscillation can be achieved. An oscillating state is often recognized by its frequency spectrum, and the Q value in such a case is defined as: Q=f0/Δf, where f0 represents the oscillation frequency, and Δf represents the half-value width of the oscillation peak of the frequency spectrum. An experiment to detect an oscillating state of a spin-torque oscillator is normally carried out by measuring the frequency spectrum with a spectrum analyzer.
In a case where a spin-torque oscillator is formed with a CPP-GMR film (hereinafter also referred to as a GMR oscillation device), a nonmagnetic metal layer is used as the spacer layer of the magnetoresistive element unit. Through experiments, it is known that a GMR oscillation device can have oscillations of approximately Q˜10 GHz/1 MHz˜104 (see “Current-driven microwave dynamics in magnetic point contacts as a function of applied field angle”, W. H. Rippard, et al., Physical Review B 70, 100406(R), (2004), for example). Accordingly, in terms of Q value, a GMR oscillation device has the same performance as or higher performance than the performance of an oscillator circuit using a crystal oscillator as a resonator. In a GMR oscillation device as a metal artificial lattice entirely formed with metal materials, the reason that a high Q value can be obtained is that a current having high current density can be supplied. It is known that the peak half-value width Δf of the frequency spectrum is inversely proportional to the square of a current I. Accordingly, the half-value width Δf becomes very small as a current with high current density is supplied. Thus, a high Q value can be achieved.
The high Q value is an advantage of a GMR oscillation device, but small oscillation output power P is a disadvantage of the GMR oscillation device. An output from a single GMR oscillation device is a very small amount of power of the order of nanowatts, which is far from a practical power level of microwatts, and is not suitable for practical use. The reason that an output from a GMR oscillation device is a small amount of power of the order of nanowatts is that a GMR device has a low magnetoresistance ratio (MR ratio) of several percents at most. There has been a suggestion to arrange GMR oscillation devices in an array, so as to obtain higher outputs (see “Mutual phase-locking of microwave spin torque nano-oscillators”, S. Kaka, et al, Nature 437, 389 (2005), for example). To obtain microwatt outputs in the case where GMR oscillation devices are arranged in an array, however, at least several tens of single GMR oscillation devices need to be arranged in an array, and all the single GMR oscillation devices need to be synchronized. This seems to lead to difficulties in device manufacture.
In a case where a spin-torque oscillator is formed with a magnetic tunnel junction film (hereinafter referred to as a TMR oscillation device), a tunnel barrier is used as the spacer layer of the magnetoresistive element unit. In recent years, high-quality magnetic tunnel junction films with low resistance and high MR ratio have been developed, and are expected to be applied to spin-injection magnetic random access memories (Spin-RAM). Through experiments, it is known that the MR ratio of a TMR (MgO-TMR) film having a tunnel barrier made of MgO (magnesium oxide) reaches several hundreds %. A TMR oscillation device can have a large oscillation output power P, thanks to the high MR ratio. Actually, the oscillation output power of spin-torque oscillators formed with MgO-TMR films is becoming closer to the practical microwatt power level, and the presently reported maximum power is about 0.16 microwatts. In a spin-torque oscillator having a magnetic tunnel junction film such as a MgO-TMR film, however, a current with high current density cannot be supplied as in a GMR oscillation device, because there are possibilities of a dielectric breakdown of the tunnel barrier. As a result, a high Q value cannot be realized. The half-value width Δf of each TMR oscillation devices observed through experiments as of today is approximately 100 MHz. Accordingly, the Q value is approximately 102, and oscillations of spin-torque oscillators formed with magnetic tunnel junction films are very unstable.
In a TMR oscillation device, a magnetization oscillation cannot be caused more often than not. This is also because of a dielectric breakdown of the tunnel barrier. As described above, a magnetization oscillation is caused in the magnetization free layer by a spin transfer effect, only if the current I is larger than the threshold current Ic (I>Ic). Where the current I is smaller than the threshold current Ic (I<Ic), a dielectric breakdown is often caused.
As described above, a GMR oscillation device and a TMR oscillation device each have an advantage and a disadvantage. The advantage of a GMR oscillation device is a high Q value, and the disadvantage is small oscillation output power. The advantage of a TMR oscillation device is large oscillation output power, and the disadvantage is a low Q value.
A spin-torque oscillator that is suitable for practical use in a small-sized microwave source, a resonator, a magnetic sensor, or the like is a spin-torque oscillator that has the above mentioned advantages of both a GMR oscillation device and a TMR oscillation device, or a spin-torque oscillator that is stable (having a high Q value) and has a high output (or large oscillation output power).